The present invention relates to geophysical exploration devices and apparatus, and particularly though not exclusively to downhole seismic tools to be lowered into a borehole extending into the earth and temporarily clamped in the borehole at a desired location for receiving waves travelling through the underground formations for analyzing the underground structure.
Downhole seismic exploration tools for use in geophysical exploration are well known in the art. Such a tool is generally elongated in shape so as to be moved up and down through a borehole drilled into the earth and typically includes a seismic detector, which is often referred to as a geophone, and a mechanical clamper for temporarily clamping the tool to the wall of the borehole at desired locations so as to pick up the seismic waves or disturbances travelling through the earth. When geophysical exploration is to be carried out with such a downhole seismic exploration tool, the tool is first lowered into the borehole using a cable and temporarily clamped to the wall of the borehole at a desired depth. Seismic waves or disturbances are created, for example by an air gun, and are picked up after travelling through the earth by the seismic detector provided inside the tool which is now temporarily clamped to the wall of the borehole. Then the tool is unclamped, moved to another depth and again temporarily clamped to the wall of the borehole for another measurement. In this manner, the tool is vertically traversed along the borehole and measurements are taken at different depths while having the tool temporarily clamped to the wall of the borehole.
A typical prior art downhole seismic exploration tool is extremely large in size, in particular in length, and can be as long as 6 meters and weigh as much as 17 kilograms. This mainly stems from the fact that the prior art tool uses a mechanical clamping device. As noted above, the tool must be temporarily clamped to the wall of the borehole, and for this purpose the tool must be provided with a clamping device. In the prior art tool, use is normally made of a mechanical clamping device having pivotally movable arms. The use of such a mechanical clamping device necessarily makes the tool large in size and weight.